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encoding/gob: Go 1.2 cannot decode Go 1.1 gobs containing net.IP #6760

rsc opened this issue Nov 13, 2013 · 5 comments

encoding/gob: Go 1.2 cannot decode Go 1.1 gobs containing net.IP #6760

rsc opened this issue Nov 13, 2013 · 5 comments


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@rsc rsc commented Nov 13, 2013

In Go 1.2, encoding/gob respects the new encoding.TextMarshaler and
encoding.TextUnmarshaler methods, and net.IP adds those methods.

Unfortunately, this means that the gob encoding of a net.IP changes from Go 1.1 to Go
1.2. That by itself might be okay: I don't believe people expect that Go 1.1 will be
able to read Go 1.2 gobs.

However, gob is also a bit picky about the decoding: if the type has a TextUnmarshaler,
gob expects to get the TextMarshaler version of the encoding and does not accept an
encoding of the underlying data. 

A Go 1.1 gob encoding of a net.IP will send the gob encoding of a standard []byte. Go
1.2 gob will refuse to decode that form.

The only other type we have added a MarshalText or MarshalBinary method to is time.Time.
But time.Time already had a GobEncode and GobDecode methods, so its behavior with gob
does not change. It appears that net.IP is the only affected type in the standard


1. Do nothing; allow Go 1.1 and Go 1.2 to be mutually unintelligible in gob when there
is a net.IP involved.

2. Change gob to accept the concrete data representation during decode, so that Go 1.1
can talk to Go 1.2 but not vice versa.

3. Change gob to ignore MarshalText/UnmarshalText, allowing only

4. Remove net.IP's MarshalText/UnmarshalText methods, breaking encoding/json's support
for net.IP (new in Go 1.2).

5. Replace net.IP's MarshalText/UnmarshalText methods with MarshalJSON/UnmarshalJSON, so
that we keep encoding/json working well for Go 1.2. (Of course, the new generic methods
were designed explicitly to avoid making package net mention JSON, so it is a bit of a
shame not to use them.)

I am leaning toward 5, which seems like the most limited change.

Rob, what do you think?
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@bradfitz bradfitz commented Nov 13, 2013

Comment 1:

Other possibility: changing encoding/gob to special-case the net.IP type, if that's the
only type affected.  But that potentially means adding a dependency on net from gob.
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@rsc rsc commented Nov 13, 2013

Comment 2:

#5 is worse than I thought, because we'd also have to put XML methods in, and you can't
do that without importing encoding/xml, which we absolutely won't do. That was the even
bigger reason for the new generic methods.
Now leaning toward #3.
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@minux minux commented Nov 13, 2013

Comment 3:

i think #3 or #4 is better.
so you won't end up with something we must  regretfully support forever.
making net relying on encoding/json is not ok IMO.
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@rsc rsc commented Nov 14, 2013

Comment 4:

This issue was closed by revision 7dd086e.

Status changed to Fixed.

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@adg adg commented Nov 18, 2013

Comment 5:

This issue was closed by revision a30cab4951a3.

@rsc rsc added fixed labels Nov 18, 2013
@rsc rsc added this to the Go1.2 milestone Apr 14, 2015
@rsc rsc removed the go1.2 label Apr 14, 2015
adg added a commit that referenced this issue May 11, 2015

««« CL 22770044 / 23fc3139589c
encoding/gob: do not use MarshalText, UnmarshalText

This seems to be the best of a long list of bad ways to fix this issue.

Fixes #6760.


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